Tweak layout of HelloThere calls to action (#3046)

should only be at most 4 images per row, centered

the visuals of seeing 6 in the first row and 2 in the second is rather displeasing

editions/tw5.com/tiddlers/hellothere/HelloThere.tid

@@ -17,7 +17,7 @@ Use it to keep your [[to-do list|TaskManagementExample]], to plan an [[essay or
 
 Unlike conventional online services, TiddlyWiki lets you choose where to keep your data, guaranteeing that in the decades to come you will [[still be able to use|Future Proof]] the notes you take today.
 
-<div style="font-size:0.7em;text-align:center;margin-top:3em;margin-bottom:3em;">
+<div style="font-size:0.7em;text-align:center;max-width:800px;margin:3em auto;">
 <a href="http://groups.google.com/group/TiddlyWiki" class="tc-btn-big-green" style="background-color:#FF8C19;" target="_blank" rel="noopener noreferrer">
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